bug 1218576 - Support remote accumulation via JS histograms. r=gfritzsche
The JS histograms, too, need to dispatch their accumulations from child to
parent.
JSHistograms_Add now only supports histograms that are in gHistogramsMap or
that were created in the parent process. After bug 1288745, maybe we'll be able
to change this to be less convoluted.
MozReview-Commit-ID: 3qTH89YKbGP